This invention relates to the control of guided missiles, and, more particularly, to an approach for controlling the guidance fins of such missiles.
Most guided missiles are controlled and stabilized with movable control surfaces or fins that project from the sides of the missile, typically near its rearward end. The fins, or possibly only a portion of the fins in larger missiles, are normally of symmetrical cross section and are pivotably mounted in the airstream. When each fin is oriented parallel to the airstream, there is no control force exerted on the missile. By pivoting the fins to be oriented at an angle with respect to the airstream, there is a resulting control force exerted on the missile and its direction or roll orientation is changed.
Some missiles may fly as fast as several times the speed of sound, and therefore control movements of the fins must be accomplished quickly and smoothly in response to a control signal. Control operations and consequent movements of the fins may be updated continuously by the missile electronics or commanded as often as several thousand times per second by a digital computer. The actuator mechanism which converts the electrical command signals to physical movement of the control fins must respond at high rates to maintain the maneuverability and stability of the high speed missile, minimizing dynamic behavior which might otherwise cause the fin not to follow the command exactly.
Two types of fin actuator systems are generally in use today. They are electromechanical systems and fluidic systems. In the former, command signals are translated to physical movement by a sophisticated electric motor, typically with a precision gear train. In the latter, which include both hydraulic and pneumatic systems, the command signal controls pressurizing valves and release valves that regulate the pressure in a cylinder with a movable piston, causing the piston to slide back and forth within the cylinder. A push rod extends out of the cylinder and is connected to a control fin output shaft upon which the fin is mounted.
Each type of actuation system, while operable in some conditions, has its drawbacks. The electromechanical system is considered to offer the higher response capability, but it may be costly, electronically and mechanically complex, difficult to build, difficult to calibrate and test, and lacking in reliability in some applications. Due to the nature of motor control, electromechanical actuation systems have certain inherent performance limitations under high fin torsional loads that may be more successfully accommodated by fluidic systems. The hydraulic and pneumatic systems can meet response requirements up to 100 cycles per second only if very precise internal tolerances are maintained, and if sophisticated valve, seal, and mechanical arrangements are devised. Even then, these systems tend to be more sensitive to nonlinear effects such as friction and backlash. The entrapped fluid in the hydraulic systems is often subject to leakage over long periods of storage, which makes periodic maintenance necessary.
The control actuator must be operable over a wide range of environmental conditions, including temperature, vibration, acceleration, and high structural and fin loadings. For example, some military specifications require that the missile be storable for extended periods and thereafter operable over temperatures ranging from as low as -65.degree. F. to as high as +190.degree. F. The actuator for the control surfaces must be made of materials that achieve satisfactory strength and other properties over the entire environmental range, and additionally must retain its performance in all specified environments.
Because of the inability of conventional pneumatic systems to meet the most demanding performance requirements over widely varying conditions, electromechanical actuators are most widely used today in high-performance missile control systems. However, as indicated, they tend to be costly, complex, prone to breakdown and performance anomalies, and difficult to test. There is therefore a need for an improved actuator system that has acceptable performance responses as well as low cost and good reliability over a range of operating conditions. The present invention fulfills this need, and further provides related advantages.